The invention relates generally to optics, and more particularly to an imaging system wherein noise filtering techniques are employed to increase the signal to noise ratio used in analyzing the image.
In many biological systems, a method of preference for identifying the location, shape, or other characteristics of a tissue is to shine light or some other form of radiation onto the tissue and observe either how the light is reflected, absorbed, refracted, or scattered by the tissue. Inherent to observing the radiative effect on said tissue is the ability to detect a signal emanating from the tissue as a consequence of disturbing the tissue in some designated fashion. Many biological structures, however, are very sensitive to light or other forms of radiation thus limiting the threshold of signal that can be used to generate nondamaging responses. Even more troublesome is that many biological tissues have weak reflectivity or weak scattering properties so that the pertinent signal resulting from the disturbance is difficult to detect. This presents the observer with the compound problem of having to detect weak signals and low signal to noise ratios.
This is illustrated herein by discussion of the anterior surface of the eye's cornea known as the epithelium, although this discussion applies to any reflecting surface within the eye. When trying to define the surface topography of the epithelium, it is often desirable to shine point sources of light onto the epithelium from a precisely established location and to measure very accurately on a detector the location of the reflected image from said point source. However, point sources are a distribution of energy about some given location and said distribution is not generally constant or uniform from source to source. Since these energy distributions are reflected from unspecified surfaces at unpredetermined angles of incidence and with varying reflectivity and scattering, the identification of the reflected images of the point sources can be a difficult problem, especially when low illumination levels are desired.
The accuracy of the technique used by Sklar et al. to measure the surface profile of the cornea (U.S. patent application Ser. No. 456,109, now U.S. Pat. No. 5,054,907) hinges on the ability of the apparatus to measure the location of the rays from point sources in front of the eye as they are reflected from the cornea. As described by Sklar et al., the reflected rays are detected by an imaging device such as a CCD camera and digitized using a frame grabber card in a computer. Other perimeter devices used to measure eye features also rely upon accurately measuring the response of the eye to shining light onto the eye. In general, the observed reflections of these spots of light are embedded within a nonuniform background which may be very close to the noise floor level for contrast selectivity or threshold filtering because of poor light source edge definition and because of low reflectivity from the corneal epithelium.
The process of separating the spots of reflected signal from the background light and locating the peak or center of the signal presents a challenging image processing problem which can be crucial to the success of the entire surface profiling procedure. The apparatus and methods for obtaining the accurate location of such signals are the subject matter of this invention. These techniques are by no means restricted to ophthalmic applications, or even biological systems. These types of techniques have been in use in military applications where targets are at times difficult to differentiate from the background field and where filtering techniques such as thresholding and fast Fourier transform filtering prove unsuccessful. In medicine and, in particular, in surgery, computers have only recently begun to be incorporated as part of surgical devices. Since the techniques that are the subject of this invention require substantial calculation, these techniques have only recently become possible in a surgical environment.
As described further below, the apparatus and methods that are the subject of this invention use nonlinear filtering techniques of mathematical morphology many of which originated with the pioneering work of J. Von Neumann for developing automated devices to analyze images by comparing a given pixel of the image with its immediate neighbors. Many of these nonlinear filtering techniques are described by Serra (Image Analysis and Mathematical Morphology, Jean Serra, Academic Press 1982) and borrow from the fields of algebraic topology, harmonic analysis, stochastic processes, integral geometry, and others. We apply these nonlinear filtering techniques of mathematical morphology to the problem of isolating and identifying the centers of reflected point light sources. In the case of the invention described by Sklar et al. (U.S. patent application Ser. No. 456,109, now U.S. Pat. No. 5,054,907) the purpose for identifying said reflected light sources, herein after referred as "points", is to present the data to a surface profiling algorithm to determine the topography of the corneal surface. In other uses of the present invention not only different structures are to be described, but different types of image sources and scattering as well as absorptions, reflections, or refractions can be considered.
The problem of identifying the location of the centers of the points is addressed in three stages. First, the points must be isolated. Then we must look for and recognize symmetry in the pattern of points. And finally, we have rejection of noise pulses which have eluded previous elimination criteria.